U.S.-backed militias in Syria said they came under attack Saturday from Russian jets and Syrian government forces in Deir el-Zour province, a flashpoint in an increasingly complex battlefield.
The Syrian Democratic Forces (SDF), an alliance of Kurdish and Arab militias fighting with the U.S.-led coalition, said six of its fighters were wounded in the strike.
The Pentagon said Russia bombed a position east of the Euphrates river where it knew SDF fighters and coalition advisers were stationed. The jets did not injure coalition forces, it said.
There was no immediate comment from Moscow or Damascus. Washington and Moscow support separate offensives in the Syrian conflict, with both advancing against Islamic State militants in the eastern region that borders Iraq.
"Our forces east of the Euphrates were hit with an attack from the Russian aircraft and Syrian regime forces, targeting
our units in the industrial zone," the SDF said in a statement.
The SDF accused Damascus of trying to obstruct its fighters. Such attacks "waste energies that should be used against terrorism ... and open the door to side conflicts," it said.
The assaults by the Russian-backed Syrian army and the U.S.-backed SDF have at times raised fears of clashes that could stoke tensions between the competing world powers.
Both offensives have converged on Islamic State from opposite sides of the Euphrates river that bisects oil-rich Deir el-Zour, Islamic State's last major foothold in Syria.
Syrian troops with Iran-backed militias have closed in from the west since last week, while the SDF advances from the east.
Russian and U.S. battles against Islamic State in Syria have mostly stayed out of each other's way, with the Euphrates often acting as a dividing line. Talks have been under way to extend a formal demarcation line, officials have said.
The U.S. coalition has said the SDF does not plan to enter Deir el-Zour city, where Syrian soldiers have broken an Islamic State siege that lasted since 2014.
Still, in June, the SDF accused the Syrian military of bombing its positions in Raqqa province, and the United States shot down a government warplane in Syria's crowded airspace.
Amed Abu Khawla, the commander of the SDF's Deir el-Zour military council, said Russian or Syrian fighter jets flew in from government territory before dawn on Saturday.
The warplanes struck as the SDF waged "heated and bloody battles" in the industrial zone on the eastern bank, seizing factories from Islamic State militants, he said.
"We have requested explanations from the Russian government," he told Reuters. "We have asked for explanations from the coalition ... and necessary action to stop these jets."
The air raid came a day after Khawla said his fighters would not let Syrian government forces cross the Euphrates. On Friday, he warned the army and its allies against firing across the river, which he said they had done in recent days.
The Russian Foreign Ministry said units of the Syrian army had already crossed.
Syria's 'strategic intent'
A senior aide to President Bashar al-Assad said the government would fight any force, including the U.S.-backed fighters, to recapture the entire country.
"I'm not saying this will happen tomorrow ... but this is the strategic intent," Bouthaina Shaaban said in a TV interview.
Syrian troops and allied forces captured villages on the western bank of the Euphrates on Saturday, state media said.
Shi'ite militias fighting with Damascus also launched attacks against Islamic State in the south of Deir el-Zour province along the border. Just over the frontier inside Iraq, the military said Iraqi armed forces dislodged Islamic State from the natural gas-rich Akashat region.
The "caliphate" that Islamic State said in 2014 it had established, spanning both countries, effectively collapsed in July when an Iraqi offensive captured the city of Mosul. In Syria, the militants have lost much of their headquarters in Raqqa to the SDF with the help of airstrikes and special forces from the U.S.-led coalition.